Gas discharge lasers, such as excimer lasers, are used in industrial applications. These applications include use in stepper systems for ultra large scale integrated circuit manufacturing. In such industrial applications it is extremely important to precisely control laser beam pulse energy and wavelength in order to ensure consistent processing quality for each wafer. Laser beam quality is critically dependent on accurate and precise control of gas mixture and pressure in the laser discharge chamber.
FIG. 1 shows an excimer laser system used as a stepper system illumination source. Gas control unit (or gas module) 101 in laser system 102 is subject to testing by the present invention. Laser system 102 produces laser output beam L used by stepper 103. Stepper control unit 104 uses a signal to trigger laser control unit 105 to generate a laser pulse. Laser control unit 105 then signals power source 106 to provide a controlled voltage pulse to lasing unit 107. Lasing unit 107 comprises a laser chamber, optical resonator, and other conventional laser beam generation components. Lasing unit 107 sends signals to laser control unit 105 that indicate the status of lasing unit 107 components and gas mixtures.
The laser chamber in lasing unit 107 is filled with a laser gas having a precise pressure and mixture. Gas mixtures are typically krypton-fluorine or other conventional rare gas/halide laser gas mixtures. Control circuits excite the laser gas mixture by applying a voltage discharge pulse of predetermined width and interval across electrodes (not shown). The voltage discharge pulse excites an oscillation in the resonating chamber and thereby creates a laser beam.
Gas control unit 101 helps to ensure constant laser beam energy and bandwidth by replacing laser chamber gases consumed during laser beam generation. Gas control unit 101 receives control signals from control unit 105 to supply the proper gas mixture and pressure to lasing unit 107. Laser control unit 105 receives signals from gas control unit 101 indicating operating status and gas pressures in gas control unit 101. Gas control unit 101 must limit mass flow rates and control the mixture ratio of gases supplied to the laser chamber. In addition, gas control unit 101 must provide a capability for handling dangerous gases, such as fluorine, typically used in gas discharge lasers. And, gas control unit 101 must provide for gas evacuation from the laser beam generation equipment in lasing unit 107 under both normal and emergency conditions.
To Applicants' knowledge, no procedures or equipment were developed to characterize and test the critical gas control unit 101 prior to the present invention. In addition, no single piece of test equipment existed that was capable of performing a comprehensive test of a gas module such as gas control unit 101.
The challenge, therefore, was to create a test fixture and evaluation methods capable of ensuring proper gas control unit function during production operations using the laser beam. A further challenge was to create a test fixture and evaluation methods that allow gas module tests and measurement for use during engineering development.